The studies described in this proposal are designed to provide basic information concerning the transepithelial transport pathways and the mechanisms by which water and solutes are absorbed in the proximal mammalian tubule and the ways in which these processes may be altered by varying physiological and pathological states. Our studies are divided into three projects and will be performed using the isolated, perfused rabbit tubule technique. Our first project will examine the inhibition of neutral transcellular active NaC1 transport induced by peritubular protein removal. Initially, we will further characterize the protein effect by investigating its colloidal specificity, its concentration dependence, its effect on cell volume and its effect on intracellular chloride ion activity. Then, we will investigate the mechanisms of NaC1 transport involved in the protein effect, placing special emphasis on the characteristics of transport at the individual cell membranes, i.e., NaC1 and KC1 symporters and Na+ H+ and C1- HCO3- antiporters. Our second project will examine the relative contribution of the transcellular and the transjunctional transport pathways to transepithelial water flow. We will differentiate between water transport through large junctional pores and water transport through small pores by comparing the diffusive to the osmotic water permeability, by determining the activation energies for diffusive and osmotic water movement, and by estimating the osmotic water permeability of the large junctional pores. Then, we will measure the osmotic water permeabilities of the individual cell membranes of proximal tubules using the crimped tubule technique, isolated cell suspensions, and brush-border membrane vesicles. For comparison, the osmotic water permeabilities will be determined for the water impermeable thick ascending limb cell membranes. Our third project will examine the electrical characteristics of acidification. We will investigate the mode of movement for bicarbonate from cell to peritubular fluid, placing special emphasis on assessing the possibility for a conductive or a neutral (KHCO3 symporter or C1-HCO3- antiporter) step. Then, we will investigate the means by which electroneutral acidification can influence the organic solute coupled sodium transport PD, placing special emphasis on acidification-induced changes in transepithelial resistance and transepithelial current flow.